U.S. Pat. No. 4,157,890 concerns a wall burner and the object is to reduce NO.sub.x by introducing combustion products into the combustion zone by aerodynamic means instead of by using cumbersome equipment to recirculate furnace flue gas from the stack back to the burner. This is done by means of staging of fuel, not staging of air, that is by the use of a preliminary or secondary burner upstream of the primary burner, in which a small fraction of the total gaseous fuel is burned in the midst of the flow of secondary air, so that the products of complete combustion of a fraction of the gases are carried by the secondary air downstreamwardly into the combustion zone of the primary burner. It may be noted that the secondary air passes through the space between the wall and the burner tube, surrounding it and passing in proximity to all the burners so that this air is provided at the place where the primary burning is initiated.
U.S. Pat. No. 3,684,189 shows conventional means for inspiration of primary air in a premix burner, generically termed a jet eductor. In this arrangement, at the upstream end of the burner tube, high pressure fuel gas contained in a pipe flows through an orifice into the entry section of a venturi, for inspirating primary air into the opening therebetween to mix with the fuel gas. U.S. Pat. Nos. 3,684,424 and 3,940,234 show a typical configuration in which a ceramic member or tile surrounds the distal or downstream end section of the burner tube and secondary air flows through a passageway between the tile and the tube.
U.S. Pat. No. 3,267,984 discloses a raw gas burner the object of which is to have the burning fuel move along an annular surface of a ceramic structure. The burner tip is provided with discharge apertures for liquid fuel as droplets and also with discharge ports for gaseous fuel. Air at relatively high pressure is supplied and flows in two paths. The major portion of the air is introduced downstream of the tip in a manner to set up a spinning mass of air into which the liquid fuel droplets are drawn by the low pressure developed in the whirling air. A minor portion of the air mixes with the gaseous fuel. This mixture provides a stable flame and the burning gaseous fuel moves downstream into the whirling air mass.
The patents discussed are incorporated herein by reference.
In U.S. Pat. No. 4,004,875 a burner for lowering NO.sub.x is disclosed which has staged secondary air, but is not a premix burner and requires recirculation of a portion of the combustion products resulting from the burning of the fuel with primary air. It also suggests that tertiary air can also be used.
U.S. Pat. No. 4,257,763 relates to U.S. Pat. No. 4,004,875 and provides a control mechanism for fixing the ratio of primary-secondary air/tertiary air. However, this does not make total air flow change with fuel flow. The patent also employs water atomization to the first burning zone.
Other patents of general interest are: U.S. Pat. Nos. 3,663,153; 3,918,834; 4,082,497; 4,439,137; and 4,289,474.